This invention relates to a process for manufacturing iron or steel alloyed with nickel. More particularly, at least some of the Ni alloying units of stainless steels are obtained by the addition of a sulfur-bearing nickel concentrate to molten iron. The process capitalizes on the presence of under-utilized slag present during refining of the iron bath, with the slag being capable of removing and holding sulfur when the bath and slag are vigorously mixed under reducing conditions.
It is known to manufacture nickel-alloyed stainless steel by melting a charge containing one or more of Ni-containing scrap, ferronickel or nickel shot in an electric arc furnace. After melting of the charge is completed, the molten iron is transferred to a refining vessel where the bath is decarburized by stirring with a mixture of oxygen and an inert gas. Additional metallic nickel, ferronickel or shot may be added into the bath to meet the nickel specification.
Ni units contained in scrap are priced about the same as Ni units in ferronickel and constitute the most expensive material for making nickel-alloyed stainless steel. Ni units in ferronickel or nickel shot are expensive owing to high production costs of liberating nickel from ore generally containing less than 3 wt. % Ni. Nickel ores are of two generic types, sulfides and laterites. In sulfur-containing ores, nickel is present mainly as the mineral pentlandite, a nickel-iron sulfide that may also be accompanied with pyrrhotite and chalcopyrite. Sulfur-containing ores typically contain 1-3 wt. % Ni and varying amounts of Cu and Co. Crushing, grinding and froth flotation are used to concentrate the valuable metals and discard as much gangue as possible. Thereafter, selective flotation and magnetic separation can be used to divide the concentrate into nickel-, copper- and iron-rich fractions for further treatment in a pyrometallurgical process. Further concentration of nickel can be obtained by subjecting the concentrate to a roasting process to eliminate up to half of the sulfur while oxidizing iron. The concentrate is smelted at 1200.degree. C. to produce a matte consisting of Ni, Fe, Cu, and S, and the slag is discarded. The matte can be placed in a converter and blown with air to further oxidize iron and sulfur. Upon cooling of the matte, distinct crystals of Ni--Fe sulfide and copper sulfide precipitate separately according to the dictates of the Fe--Cu--Ni--S phase diagram. After crushing and grinding, the sulfide fraction containing the two crystals is separated into copper sulfide and Ni--Fe sulfide concentrates by froth flotation. The Ni--Fe sulfide concentrate undergo several more energy-intensive stages in route to producing ferronickel and nickel shot. The Ni--Fe sulfide can be converted to granular Ni--Fe oxide sinter in a fluidized bed from which a nickel cathode is produced by electrolysis. Alternatively, Ni--Fe concentrates can undergo a conversion to Ni and Fe carbonyls in a chlorination process to decompose into nickel and iron powders.
It is known to produce stainless steel by charging nickel-bearing laterite ore directly into a refining vessel having a top blown oxygen lance and bottom tuyeres for blowing stirring gas. Such ores contain at most 3% Ni, with over 80% of the ore weight converting to slag. U.S. Pat. No. 5,047,082 discloses producing stainless steel in an oxygen converter using a low-sulfur nickel-bearing ore instead of ferronickel to obtain the needed Ni units. The nickel ore is reduced by carbon dissolved in molten iron and char present in the slag. U.S. Pat. No. 5,039,480 discloses producing stainless steel in a converter by sequentially smelting and reducing low sulfur nickel-bearing ore and then chromite ore, instead of ferronickel and ferrochromium. The ores are reduced by carbon dissolved in the molten iron and char present in the slag.
Because laterite ore contains little sulfur, the bulk of Ni units for making stainless steel can come from the ore. However, the large quantity of slag accompanying the Ni units necessitates a separate, energy-intensive smelting step in addition to the refining step, requiring increased processing time and possibly a separate reactor.
Control of bath sulfur content is one of the oldest and broadest concerns during refining of iron. Ever since iron was smelted in the early blast furnaces, it was known that slag in contact with molten iron offered a means for removing some of the sulfur originating from coke used as fuel. More recently, key factors identified for sulfur removal during smelting include controlling slag basicity as a function of partial pressures of gaseous oxygen of the slag and controlling slag temperature.
Nevertheless, the slag sulfur solubility limit normally is not reached during routine refining of stainless steel alloyed with nickel because the total sulfur load in the refining vessel originating from melting the solid charge material in an electric arc furnace is low. Hence, slag desulfurization capacity in the refining vessel is under-utilized. Increased slag weight, the presence of residual reductants in the bath and the manipulation of slag composition can all increase this degree of under-utilization. There also remains a long felt need for lowering the cost of nickel alloying units used in the manufacture of alloyed iron or steel such as nickel-alloyed steel and austenitic stainless steel without the need for major capital expenditure.